Generally, an insert is fastened to a cutting tool mounted on a machine tool, to be used for cutting a workpiece composed of ferrous, nonferrous metals, nonmetal materials, and so on.
Such insert generally includes a body, a cutting portion provided on an upper surface edge of the body to directly cut the workpiece, and a fastening portion provided on the body to be fastened to a holder of the cutting tool. Specifically, the cutting portion includes a fastening hole being passed through the body from upper surface to lower surface. For reference, the insert is also called as “indexable insert” as it is replaceable through the fastening hole.
Meanwhile, the tool holder generally includes two or more chip pockets having a pocket-shaped flute to discharge chips, a sheet portion provided at each of the chip pockets to fasten the insert, and a web forming a boundary between the first and second pockets.
One of related insert 10 is disclosed in U.S. Pat. No. 6,053,671, as illustrated in FIGS. 13 to 15, which includes a circular upper surface 11, a circular cutting edge 16 forming an upper surface edge, a lower surface 12 seated on the sheet portion 32 of the tool holder 30, an edge surface connecting the upper surface 11 with the lower surface 12 and being composed of an upper edge surface 13a in contact with the upper surface 11, and a lower edge surface 13b in contact with the lower surface 12, and a step 18 formed between the upper edge surface 13a and the lower edge surface 13b. Further, the upper edge surface 13a having a frustoconical shape with no pattern formed on an outer circumference thereof, and the lower edge surface 13b has eight planes 13b′ along a circumference thereof to prevent rotation of the insert 10, and when seen from below, has an octagonal shape (see FIG. 15).
However, in order to provide more clearance angle of the circular cutting edge while maintaining the size of the circular cutting edge as is, as illustrated in FIG. 14, the related technology requires a greater rake angle of the entire frustoconical upper edge surface 13a with respect to the central axis 20, but this in turn relatively reduces the volume and deteriorates rigidity compared to when the rake angle is smaller.
Further, as illustrated in FIG. 13, the upper edge surface 13a is in a position where it is rotatable such that only the lower edge surface 13b prevents the rotation of the insert 10. Accordingly, during machining of the workpiece by the insert 10, torsional stress is focused in between the upper edge surface 13a and the lower edge surface 13b, thus generating breakage, and so on. As a result, the problem of shortened life of the insert 10 occurs.
Further, as illustrated in FIGS. 13 to 15, since eight planes 13b′ forming the lower edge surface 13b are placed parallel with the central axis 20, that is, since the structure is missing a support to the force with which the insert 10 escapes in the central axis 20 direction, during workpiece machining by the insert 10, the problem of raising of the tool holder 30 from the sheet portion 32 occurs.